1. Field of the Invention
The present invention relates to a magnetic resonance scanner with a magnetic resonance system comprising at least two radiofrequency transmitter/receiver units which each have a radiofrequency transmitter/receiver coil, a radiofrequency current source which can be coupled to the radiofrequency transmitter/receiver coil for the transmission, and an amplifier (5) which can be coupled to the radiofrequency transmitter/receiver coil for the reception. According to a second aspect, the invention relates to a method for controlling a magnetic resonance scanner.
2. Background Description
The requirements for modern magnetic resonance systems for magnetic resonance imaging (MRI) and magnetic resonance spectroscopy (MRS) necessitate the use of radiofrequency units which have a plurality of coil elements. In this context, the utilized radiofrequency coils are conventionally referred to as radiofrequency antennas. Here, combinations of resonant radiofrequency coils and/or radiofrequency antennas are frequently used, with the individual antennas being excited by alternating voltage sources.
In order to generate the well-defined radiofrequency magnetic field in a sample body, e.g. in a patient in magnetic resonance imaging, required for the magnetic resonance experiment, the current has to be monitored or controlled in each individual antenna element. However, this current control is hindered by mutual coupling of the individual antenna elements; this constitutes an increasing problem in both transmission processes and reception processes, particularly in the case of relatively high frequencies. The coupling amongst the antenna elements leads to a distortion of the sensitivity profile of the coil in the antenna element during the reception and furthermore reduces the homogeneity of the radiofrequency magnetic field during transmission.
According to U.S. Pat. No. 5,489,847, additional inductive or capacitive decoupling elements are provided for decoupling the antenna elements of a magnetic resonance array coil comprising a plurality of coils. However, a disadvantage of such solutions is that these can only be applied in certain antenna configurations. Decoupling networks proposed as an alternative thereto can only be utilized with great difficulty in practical application because this network has to be reoptimized for every object to be examined. By way of example, if utilized in magnetic resonance imaging, this generally requires a renewed optimization of the decoupling network for each individual patient to be examined.
US 2005/0242816 A1 discloses a transmitter coil array, in which the individual antenna elements are decoupled. The antenna current can be controlled in a targeted fashion due to the use of radiofrequency current sources; this makes it possible to effectively decouple the individual antenna elements from one another. Although the system described therein allows a decoupling of the individual antennas during transmission, the document provides no indication for decoupling during a reception process.
The noise of the current source used for transmission in particular leads to an unacceptable deterioration of the signal-to-noise ratio during the reception process. A further cutting limitation of the system for the use in magnetic resonance scanners is the strong impairment of the homogeneity of the static magnetic field in the system by the components which control the coil currents in a targeted fashion, and by the direct current portion of the radiofrequency current source. Finally, the provision of an antenna element array for transmission and a further array of additional antenna elements for reception is very costly as a result of the additionally required components.
DE 40 16 640 A1 discloses a transmitter/receiver switch with PIN diodes for nuclear magnetic resonance equipment. The transmitter/receiver switch is designed such that no separate direct current power supply is required. It is disadvantageous that the switch described therein does not permit a current-controlled operation of the magnetic resonance scanner and hence there are measurement errors due to couplings between radiofrequency transmitter/receiver units. U.S. Pat. No. 4,764,726 discloses a changeover switch for nuclear magnetic resonance equipment which has the same disadvantages.
US 2003/0184293 A1 discloses a magnetic resonance system which comprises a transmitter/receiver switch. The disadvantage therein lies in the fact that such a system implies the use of impedance-matched radiofrequency feed lines and separate radiofrequency amplifiers for actuating the radiofrequency coils. Thus, a current-controlled operation of the radiofrequency coil is not possible in this case either, and so scattered fields lead to measurement errors.
DE 103 25 634 A1 discloses a low-noise preamplifier for magnetic resonance technology. However, this document does not disclose how the preamplifier can be combined with a radiofrequency current source such that a current-controlled operation is possible. If there is no current-controlled operation, induced voltages can cause measurement errors.